Acute critical illness such as trauma, sepsis and resuscitated cardiac arrest affects millions of people worldwide annually with a projected 40% increase in global deaths due to injuries in the period 2002 to 2030. Approximately one quarter of acute critically ill patients develop severe hemostatic aberrations resulting in impaired clotting ability (coagulopathy); such acute critically ill patients with coagulopathy having 3-4 times higher mortality rates compared to their non-coagulopathic counterparts (40-50% vs. 10-15%). Importantly, besides early mortality related to exsanguination, acute critically ill patients with coagulopathy have a several-fold increased risk of developing and dying from multiple organ failure in the days and weeks that follow the injurious hit. Regrettably, the outcome for acute critically ill patients with coagulopathy has remained unaltered since the 1970ies, despite a general improvement in intensive care capabilities, pointing towards a lack of identification of the pathophysiologic mechanism(s) responsible for the poor outcome [Artenstein et al 2013; Marshall 2014]. Currently, no drugs/pharmacological agents or therapeutic interventions are registered to specifically treat coagulopathy in acute critically ill patients.
Microcirculatory failure is a hallmark of acute critical illness that is caused by numerous injurious hits to the vascular system, including the endothelium, i.e. the single layer of cells that lines the interior of all blood vessels in the body. Although there is emerging consensus that endothelial damage is a critical contributing factor to the development of organ failure and poor outcome in acute critically ill patients [Opal and van der Poll 2015], no drugs or therapies are currently registered to specifically treat endothelial damage in acute critically ill patients. There is thus an urgent unmet need for diagnostic tests and therapeutic interventions capable of reversing and treating the deleterious changes observed in the vascular system, including the endothelium, in acute critically ill patients [Marshall 2014; Simmons and Pittet 2015].
The Endothelium
The endothelium is the collective thin layer of cells (endothelial cells) that lines the interior of all blood and lymphatic vessels throughout the body. It is one of the largest “organs” in the human body having a total weight of approximately 1 kg and covering a total surface area of approximately 4-7,000 m2. The luminal surface of the endothelial cells is covered by a 0.2-1.0 μm thick negatively charged carbohydrate-rich surface layer, the endothelial glycocalyx, that also represents a large structure in the vascular system by containing a fixed non-circulating plasma volume of approximately 1 liter in adults, corresponding to one third of the intravascular plasma volume. The glycocalyx provides the endothelium with an anti-adhesive and anticoagulant surface that protects the endothelial cells and maintains vascular barrier function. The glycocalyx is a mesh-like structure comprising proteoglycan and glycoprotein backbone molecules that bind and incorporate various soluble molecules derived from the plasma and endothelium, with the highest amounts of plasma derived constituents towards the luminal surface.
The endothelium is critically involved in maintaining the delicate homeostasis between the circulating blood and all vital organs. By traversing each and every organ in the body, the endothelium is pivotal for maintaining homeostasis between the circulating blood and all vital organs and cells of the body and, hence, damage to the endothelium is a key factor of the observed poor outcome in acute critically ill patients. The endothelium controls vasomotor balance, vascular integrity, blood cell adhesion and trafficking, immune surveillance, inflammation and angiogenesis, and it is instrumental for balancing hemostasis through its release, expression and support of systems and elements that either promote or inhibit hemostasis.
In a healthy state, the endothelium is anticoagulated by constituents of the glycocalyx and the endothelial cells themselves. Upon endothelial damage, these constituents are released to the flowing blood while retaining their anticoagulant effects, thereby contributing to coagulopathy of acute critical illness. At the same time, the damaged endothelium becomes prothrombotic and triggers formation of microvascular thrombosis resulting in impaired oxygen delivery to tissues, organ failure and ultimately death [Johansson and Ostrowski 2010]. Furthermore, endothelial damage disrupts the inter-cellular tight junctions responsible for maintaining endothelial barrier function between the flowing blood and the tissues. This results in capillary leakage, which further drives hypotension and oxygen deprivation and thereby contribute directly to multiple organ failure and death [Opal and van der Poll 2015].
Until now, most research on acute critically ill patients with coagulopathy has been limited to studying circulating factors of single pathways such as the coagulation-, complement- and inflammatory systems readily measured in the plasma of patients. However, recognizing that the circulating plasma is only one part of the complex vascular system, which includes blood cells (platelets, leukocytes, red blood cells), microparticles and all the vessels that contain the blood, the inventors hypothesized that the observed plasma aberrations reflect a universal, evolutionary developed response to acute critical illness that is associated with concurrent changes in the vascular endothelium and circulating blood cells [Johansson and Ostrowski 2010].
Prostacyclin
Prostacyclin is a naturally occurring prostaglandin released by healthy endothelial cells. In humans, prostacyclin generation by the vascular endothelium is approximately 0.08-0.10 ng/kg/min with a maximal concentration of 3.4 pg/1 in the circulation [Davies and Hagen 1993]. Prostacyclin performs its function through a paracrine signaling cascade that involves G protein-coupled receptors (GPCR) on nearby endothelial cells and platelets.
The two main pharmacologic actions of prostacyclin and its analogues are vasodilation and inhibition of platelet aggregation, which is reflected by the current medical indications for prostacyclin analogs [including but not limited to Iloprost, Epoprostenol, Epoprostenol Sodium, treprostenil sodium, selexipag, Beraprost, etc. administered either intravenously (i.v.), subcutaneously (s.c.) or oral (p.o.)]:    1) primary pulmonary hypertension (NYHA class III-IV patients)    2) secondary pulmonary hypertension (NYHA class III-IV patients with the scleroderma spectrum of disease who do not respond adequately to conventional therapy)    3) anticoagulation during hemodialysis or renal replacement therapy when heparin is contraindicated    4) peripheral arterial disease with imminent risk of amputation when surgical treatment and angioplasty is not possible    5) progressive thrombangitis obliterans (mb. Bürger) with critical limb ischemia when surgical treatment and angioplasty is not possible
A study in human volunteers reported that prostacyclin at doses less than 8 ng/kg/min had no significant effect on systolic or diastolic blood pressure whereas a dose of 8 ng/kg/min reduced diastolic blood pressure. The study found no effect on systolic blood pressure in doses up to 16 ng/kg/min [O'Grady et al 1980]. However, several studies have reported that prostacyclin in doses from 5-10 ng/kg/min lowers systolic blood pressure dose-dependently in acute critically ill patients [Bihari et al 1987; Radermacher et al 1995].
In healthy volunteers, 1-4 ng/kg/min prostacyclin infusion did not influence blood pressure and in patients suffering from traumatic brain injury [Grande et al 2000; Naredi et al 2001], acute myocardial infarction treated by percutaneous coronary intervention (PCI) [Holmvang et al 2012], septic shock [Kiefer et al 2001; Lehmann et al 2000] or CABG surgery [Morgera et al 2002], 0.5-2 ng/kg/min prostacyclin infusion did not negatively influence blood pressure.
In conclusion it was found that prostacyclin dilates all vascular beds dose-dependently but the hemodynamic effects of low-dose prostacyclin infusion (up to 4 ng/kg/min) are negligible in healthy volunteers and in acute critically ill patients.
Inhibition of Platelet Aggregation
Prostacyclin inhibition of platelet aggregation is mediated through platelet expressed GPCR (IP), which upon prostacyclin binding signals adenylyl cyclase to produce cAMP, which activates PKA to decrease free intracellular calcium concentrations. The rise in cAMP directly inhibits platelet activation (secretion and aggregation) and counteracts increases in cytosolic calcium resulting from platelet activation by platelet agonists.
Historically, prostacyclin has been considered to be the most potent endogenous inhibitor of platelet aggregation in the human organism [Moncada et al 1976; O'Grady et al 1980]. However, as for the vasodilatory effect, the antiaggretory effect is highly dose-dependent [Moncada et al 1976; O'Grady et al 1980]. Of paramount importance for the suggested intervention, the inventors have investigated the anti-thrombotic potential of prostacyclin with functional whole blood hemostatic assays proven to correlate with clinical bleeding conditions and transfusion requirements (thrombelastography (TEG) and impedance aggregometry (Multiplate)) and surprisingly they discovered that low-dose prostacyclin infusion had no measurable anti-thrombotic effects.
In addition to the dose-dependent pharmacologic actions of prostacyclin (vasodilation and inhibition of platelet aggregation), there is emerging evidence that endogenously released prostacyclin has a paracrine cytoprotective function that is mimicked by prostacyclin analogs. This notion is considered important for the suggested intervention as acute critical illness disrupts normal prostacyclin release by endothelial cells leaving the patients with a malfunctioning systemically disrupted vascular endothelium.
Cytoprotection
The cytoprotective action of prostacyclin is mediated through prostacyclin IP receptors expressed on a broad range of cells including endothelial cells.
At the endothelial level, prostacyclin cytoprotection results in preservation and/or promotion of endothelial integrity and endothelial quiescence favoring an anticoagulant, antiadhesive, antiapoptotic and antiinflammatory phenotype of the endothelium. Prostacyclin directly promotes recruitment of endothelial progenitor cells, which enhances endothelial re-endothelilization of injured endothelium, inhibits endothelial apoptosis and prevents mitochondrial uncoupling of phosphorylation from oxidation in the respiratory chain in conditions with cellular stress, whereby mitochondrial structure and function is preserved and apoptosis reduced. Also, prostacyclin improves endothelial integrity and vascular barrier function by upregulating vascular endothelial (VE)-cadherin, which is responsible for maintaining tight junctions between endothelial cells. Finally, prostacyclin promotes endothelial quiescence by enhancing endothelial expression of other (besides prostacyclin) natural anticoagulant pathways which all, to some degree, exerts cytoprotive functions.
In addition to the direct action of prostacyclin on endothelial cells, the influence of prostacyclin on other cells and tissues indirectly protect the endothelium. Thus, prostacyclin induced vasodilation mediated through vascular smooth muscle relaxation protects the endothelium by ensuring microvascular perfusion and oxygen supply to (potentially hypoxic) cells and tissues. Also, prostacyclin stabilizes lysozomal and cell membranes in immunologic cells which reduces inflammation hereby preventing bystander activation and potential damage of the endothelium [Zardi et al 2005; Zardi et al 2007]. Finally, prostacyclin induced inhibition of platelet aggregation indirectly preserves endothelial quiescence as activated platelets promote endothelial activation.
In conclusion, prostacyclin exerts widespread cytoprotective actions that result in preservation and/or promotion of endothelial integrity and quiescence favoring an anticoagulant, antiadhesive, antiapoptotic and antiinflammatory phenotype of the endothelium thus counteracting the pathologic state of the endothelium in systemic endotheliopathic syndrome.